The Transmembrane Domain of Glycoprotein Ibβ Is Critical to Efficient Expression of Glycoprotein Ib-IX Complex in the Plasma Membrane

Abstract

Lack of expression of glycoprotein (GP)Ib Platelet glycoprotein (GP)2 Ib-IX-V complex mediates the initial tethering and rolling of platelets on von Willebrand factor (vWF) localized at the vascular injury site (1, 2). Upon ligation with the A1 domain of vWF, the GP Ib-IX-V complex transmits inward a signal that leads to activation of integrin ␣IIb␤3 and eventual platelet activation and aggregation (3, 4).Although the GP Ib-IX-V complex is widely considered to comprise GP Ib␣, GP Ib␤, GP IX, and GP V with a 2:2:2:1 stoichiometry (5-7), it is not clear how these subunits assemble into a functional receptor complex in the membrane, partly because intersubunit interactions are largely unknown.Abnormally low expression of the GP Ib-IX-V complex in human platelets can result from mutations in either GP Ib␣, GP Ib␤, or GP IX, but not GP V (8). Such phenomena can be reproduced in transfected Chinese hamster ovary (CHO) cells; missing any of the three subunits leads to no or significantly lowered surface expression of the GP Ib-IX complex (9, 10). In contrast, GP V is not required for efficient expression of the GP Ib-IX complex, although there are mixed reports on whether it enhances the complex expression (11, 12). Furthermore, it has been shown in transfected cells that the GP Ib-IX complex assembles in the endoplasmic reticulum before being transported as a single entity to the plasma membrane. Failure of proper complex assembly results in prompt degradation of GP Ib␣ in the lysosome and accumulation of nonnative GP IX inside the cell (13,14).Although the interdependence among the subunits in the GP Ib-IX complex is well documented, the underlying reasons are not clear. The requirement of all three subunits to efficient surface expression of the complex posits the importance of intersubunit interactions to the proper assembly and therefore stability of the receptor complex. Based on the effects of mutations in GP Ib␤ on the expression level of GP IX, the N-terminal cysteine knot region in GP Ib␤ was suggested to mediate the interaction with GP IX (15). It helps to explain the dependence of GP IX expression on GP Ib␤. Why the expression of GP Ib␣ is dependent on GP Ib␤ and/or IX nonetheless remains unclear.The transmembrane (TM) domains of GP Ib␣ and IX may be important to efficient surface expression of the GP Ib-IX complex. Deletion of either the extracellular or cytoplasmic domain of GP Ib␣ does not significantly affect complex expression in transfected cells (16,17). Consistently, the Ib␣ extracellular domain, but not the combined extracellular and TM domains, can be replaced with counterparts from the interleukin-4 receptor while maintaining reasonable expression level of the receptor complex (18). In addition, a single-site mutation in the IX TM domain was recently identified to cause decreased expression of the GP Ib-IX

Identification of a juxtamembrane mechanosensitive domain in the platelet mechanosensor glycoprotein Ib-IX complex

“…1-2 3 10 4 cells/mL). The supernatant containing biotinylated GPIb-IX was further analyzed by Western blot and flow cytometry largely as described previously, 30,33,34 or it was stored at 280°C for the force measurement.…”

The Transmembrane Domains of β and IX Subunits Mediate the Localization of the Platelet Glycoprotein Ib-IX Complex to the Glycosphingolipid-enriched Membrane Domain

“…2, A and B). In contrast, previous reports have shown that transient expression of mutant GP Ib␤ and GP IX whose TMDs are replaced either by polyleucine (pL, ␤ pL and IX pL ) or polyleucine-alanine (pLA, ␤ pLA and IX pLA ) residues causes nonexpression of GP Ib␣ only in mutant GP Ib␤ cells instead of mutant GP IX cells (38). Therefore, it is possible that 1) the pL or pLA type of TMD may by itself interfere with the expression of GP Ib␤ and therein GP Ib␣, and/or 2) the Trf TMD in the swapped GP Ib␤ has an improved capability of forming stable bonds with the TMDs of GP Ib␣ and GP IX to maintain a relatively higher expression of GP Ib␣ on the cell surface.…”

The dimerization interface of the glycoprotein Ibβ transmembrane domain corresponds to polar residues within a leucine zipper motif

“…Our previous studies show that the TM domain of Ibb is critical for efficient expression of the GP Ib-IX complex in the plasma membrane, because replacing the TM with poly-Leu or poly-LeuAla diminished surface expression of the GP Ib-IX complex in the plasma membrane. 8 Further studies showed that TM domain associations of Iba, Ibb, and IX domain favorably position the membrane-proximal Cys residues for disulfide bond formation, and this supports the ab 2 configuration. 5,9 All these observations suggest that the TM domains of the GP Ib-IX complex are responsible for functionally important associations of the GP Ib-IX complex in membranes.…”